The present invention relates to an improvement in the suspension polymerization of vinyl chloride or, more particularly, to an improvement in the process of suspension polymerization of vinyl chloride or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium, by which a high-quality vinyl chloride-based polymeric resin can be produced in a high efficiency.
As is well known, vinyl chloride-based polymeric resins are mostly produced by the suspension polymerization of vinyl chloride or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium in a batch-wise process. Namely, the process is conducted by introducing deionized water as the polymerization medium, suspending agent and polymerization initiator into a jacketed polymerization reactor equipped with a stirrer and a reflux condenser and, after evacuation of the reactor to exclude the atmospheric oxygen, vinyl chloride monomer is introduced into the reactor and dispersed in the aqueous medium under agitation to form a polymerization mixture in the form of a suspension which is then heated up to a polymerization temperature of, usually, 50.degree. to 60.degree. C. where the polymerization reaction starts and proceeds.
One of the problems in the above described process is that, in the course of the temperature elevation up to the polymerization temperature, bubbles rise in the liquid phase of the polymerization mixture to form foams which cover and float on the liquid surface. These foams are hardly destroyable even by undertaking a mechaical or other foam-breaking means so that the polymerization reaction proceeds in the liquid medium covered with the foams to cause various disadvantages. For example, polymer scale is heavily deposited on the inner walls of the polymerization reactor, especially, in the areas just above the interface between the liquid and gaseous phases to greatly decrease the productivity of the process. Further, the polymerization reaction proceeds also in the liquid films forming the foams to form a resin in a foamy form resulting in a decrease in the yield of the resin and quality degradation of the resin product. In addition, the resin product produced in such a foam-covered polymerization medium usually contains a large number of so-called fish-eyes which are detrimental to the quality of the resin product.
Various attempts and proposals have of course been made in the prior art to reduce generation of foams or to destroy once generated foams but none of the prior art methods is very effective.